Method of riser deployment on a subsea wellhead

ABSTRACT

The riser assembly is plugged adjacent its bottom end either as it is made up or after full makeup or anywhere in between. A heavy fluid is either added to the riser string as it is made up or delivered into the string after makeup to displace seawater or other fluids that may have been in the riser as it is assembled. The floating rig can be secured over the subsea well head and the riser now full of heavy fluid is less prone to be displaced by wave action and preferably simply dangles straight down so that the connection to the subsea wellhead can be simply accomplished. The effect of wave action is minimized or eliminated when deciding placement of the floating rig with respect to the wellhead. Repositioning of the rig takes less effort and is needed less frequently.

FIELD OF THE INVENTION

The field of this invention is riser placement on a subsea wellhead and more specifically when using a floating rig.

BACKGROUND OF THE INVENTION

Floating rigs are connected to subsea wellheads through a string called a riser that spans the distance between the rig floor and the wellhead located just above the sea floor. Floating rigs are normally positioned and secured to hold a selected position while still able to compensate for the rise and fall of the surface from wave action. Before or after a rig is positioned the riser assembly is made up in sections on the rig floor and run in. If the riser is made up before, the currents can be used to guide the rig closer to the subsea BOP. The water depths for some floating rigs can be in the thousands of meters and the wave action at the surface has a component below the surface that can deflect the riser string as it is made up and lowered toward the wellhead. The skewing effect is well known and in the past the position of the floating rig with respect to the wellhead is adjusted to take advantage of the expected lateral forces from wave action so that when the riser is fully assembled, its lower end will be in close proximity to the wellhead so that the two can be stabbed in, or simply connected with divers or remotely operated vehicles (ROVs). The problem lies in the inherent unpredictability of the tidal forces both in strength and in direction. Repositioning of the floating rig is time consuming and may require the riser string to be pulled back onto the rig before it can be repositioned.

The present invention is directed at a technique to minimize the effect of wave action as opposed to the past techniques of simply trying to compensate for its predicted force and direction. Those and other aspects of the present invention will be more readily understood by those skilled in the art from a review of the description of the preferred embodiment and the drawings as well as the claims below that define the full scope of the invention.

SUMMARY OF THE INVENTION

The riser assembly is plugged adjacent its bottom end either as it is made up or after full makeup or anywhere in between. A heavy fluid is either added to the riser string as it is made up or delivered into the string after makeup to displace seawater or other fluids that may have been in the riser as it is assembled. The floating rig can be secured over the subsea well head and the riser now full of heavy fluid is less prone to be displaced by wave action and preferably simply dangles straight down so that the connection to the subsea wellhead can be simply accomplished. The effect of wave action is minimized or eliminated when deciding placement of the floating rig with respect to the wellhead. Repositioning of the rig takes less effort and is needed less frequently.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows current deflecting a riser that is plugged at the bottom;

FIG. 2 is a close in view of the riser bottom showing the plug;

FIG. 3 shows the riser filled with a heavy fluid and generally aligned with the wellhead;

FIG. 4 shows the riser connected to the wellhead; and

FIG. 5 shows the riser connected and the plug removed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a floating rig 10 set over a subsea wellhead 12 and supporting a riser 14 that is being deflected by currents active between the sea bed 16 and the surface 18. Near the lower end 20 of the riser 14 is a plug 22 that can be installed when the riser string 14 is assembled or later after it is fully assembled by running it in using a variety of known techniques. In the event the plug 22 is initially installed in the lowermost joint or joints of the riser string 14, drilling mud or seawater, for example, can be added above the plug 22 as each stand of riser pipe is connected at the surface and lowered in. FIG. 2 shows the lower end 20 closer up showing the plug 22 and schematically illustrating the connecting hardware 24 to attach to the wellhead 12. The plug can be deployed or dropped into the riser 14 and displaced with heavy fluid to a seat within the riser 14. Alternatively, the plug can be in position at the lower end of the riser but not seated off so as to allow heavy fluid to be spotted above it displacing the seawater, after which the plug can be seated off or set, depending on the type used. The plug can be an inflatable, or mechanical or a non-elstomer type. It can also be wiper that lands on a seat in the riser 14.

FIG. 3 shows the riser 14 full of heavy fluid 26. There are many choices for this fluid with the criteria being to provide stiffness and fully take advantage of gravity to counteract the applied force from currents. In the ideal condition with the rig 10 above the wellhead 12 the riser 14 dangles straight down over the wellhead 12, as shown in FIG. 3. FIG. 4 shows the riser 14 stabbed in, or simply connected such as with divers or an ROV with the plug 22 still in place.

FIG. 5 illustrates the plug 22 released with known tools and brought to surface with flow from the well that also displaces the heavy fluid 26 out of the riser 14. Other alternatives to get to the FIG. 5 condition are contemplated. The plug can be milled or drilled with the cuttings going to the surface with the heavy fluid. Alternatively, the plug can be removed with a known technique and the heavy fluid simply pumped downhole instead of being allowed to be displaced to the surface. Alternative to heavy fluids, such as high weight drilling fluids other fluids or gels can be used as long as they are easily pumped into position and just as simply displaced out of the way or pumped out of the way. Solids can be included in the heavy fluid in various shapes such as spheres using preferably dense metals and sizing the objects so that flow from the well, for example, will bring them out to the surface.

In the end, the method allows placement of the rig 10 over the wellhead 12 with little or no concern over lateral deflection of the string 14 away from the wellhead 12 due to current force. The need to guess in advance the deflection effects of currents, which can be highly variable, is greatly reduced or eliminated. The rig 10 is unlikely to need repositioning during the procedure of running the riser 14 and connecting it to the wellhead.

The above description is illustrative of the preferred embodiment and many modifications may be made by those skilled in the art without departing from the invention whose scope is to be determined from the literal and equivalent scope of the claims below. 

1. A method of connecting a riser to a subsea wellhead comprising: assembling the riser; installing a plug in said riser: adding a heavy material in the riser above the plug; lowering said riser toward the wellhead; and connecting the riser to the wellhead.
 2. The method of claim 1, comprising: making said riser resist deflection from current forces due to said heavy material.
 3. The method of claim 2, comprising: positioning a rig closer to being over the wellhead due to reduced riser deflection from current when holding said heavy material.
 4. The method of claim 1, comprising: installing and setting said plug as said riser is assembled at the surface; filling in said heavy material as additional riser sections are added at the surface.
 5. The method of claim 1, comprising: installing said plug after said riser is fully assembled and lowered; displacing seawater in said riser with said heavy material.
 6. The method of claim 5, comprising: sealing off said riser with said plug after said displacing.
 7. The method of claim 1, comprising: using a liquid as said heavy material.
 8. The method of claim 7, comprising: including solids in said liquid.
 9. The method of claim 1, comprising: using a gel as said heavy material.
 10. The method of claim 1, comprising: removing said plug after said connecting.
 11. The method of claim 10, comprising: bringing the plug up the riser with flow from the well.
 12. The method of claim 10, comprising: drilling or milling the plug to remove it.
 13. The method of claim 1, comprising: said installing further comprises using an inflatable plug, a mechanically set plug, non-elastomer plug or a wiper plug landing on a seat. 